1. Field of the Invention
This invention relates generally to designs for parallel plate voltage-variable capacitors (varactors), and more particularly, to the design of electrode geometries for parallel plate ferroelectric varactors to improve various performance characteristics such as tuning, reliability, and electrostatic discharge, and for high field testing.
2. Description of the Related Art
Capacitors are a basic building block for electronic circuits, and voltage-variable capacitors (varactors) have the added flexibility that their capacitance can be tuned by changing a bias voltage across the capacitor. Dielectric materials that have a permittivity that depends on the applied electric field can be used to form such varactors. Varactors have an added advantage that they can be easily integrated with other components, particularly if the dielectric layer is a thin film. One common approach to voltage-variable varactors is the “parallel-plate” configuration, in which the voltage-variable dielectric is sandwiched between two electrodes. For example, in an integrated varactor, one electrode may be a bottom conducting layer, the dielectric may be a ferroelectric thin film deposited over the bottom electrode, and the top electrode may be a metal layer.
In the parallel-plate configuration, the total capacitance of the varactor is determined primarily by the parallel combination of two components: a bulk capacitance due to the direct electric field between the two electrodes and a fringing or fringe capacitance due to the fringing electric field between the two electrodes. Ideally, the bulk capacitance should dominate over the fringing capacitance so that the bulk capacitance is the major contributor to the total capacitance of the varactor. This results in greater tunability of the varactor capacitance since the bulk capacitance typically is more tunable than the fringing capacitance.
The use of thin film ferroelectric layers of barium titanate, strontium titanate, and/or composites of the two (collectively referred to as “BST” materials) has led to smaller area varactors since the permittivity of such BST layers is high. In such varactors, however, the assumption that the bulk capacitance is the major contributor to the total capacitance may no longer hold true as the fringing capacitance begins to become a significant fraction of the total capacitance. This is especially true for small capacitance value varactors, because as the area that contributes to the bulk capacitance decreases, the fringing capacitance has a greater impact on the total capacitance.
Thus, there is a need for a thin film BST varactor that has a total capacitance determined in greater part by the bulk capacitance to improve tunability of the thin film BST varactor.